This invention relates to metal catalysts for alkylene oxide polymerization.
Alkylene oxides such as ethylene oxide, propylene oxide and 1,2-butylene oxide are polymerized to form a wide variety of polyether products. For example, polyether polyols are prepared in large quantities for polyurethane applications. Other polyethers are used as lubricants, brake fluids, compressor fluids, and many other applications.
These polyethers are commonly prepared by polymerizing one or more alkylene oxides in the presence of an initiator compound and an alkali metal catalyst. The initiator compound is typically a material having one or more hydroxyl, primary or secondary amine, carboxyl or thiol groups. The function of the initiator is to set the nominal functionality (number of hydroxyl groups/molecule) of the product polyether, and in some instances to incorporate some desired functional group into the product.
Until recently, the catalyst of choice was an alkali metal hydroxide such as potassium hydroxide. Potassium hydroxide has the advantages of being inexpensive, adaptable to the polymerization of various alkylene oxides, and easily recoverable from the product polyether.
However, to a varying degree, alkali metal hydroxides catalyze an isomerization of propylene oxide to form allyl alcohol. Allyl alcohol acts as a monofunctional initiator during the polymerization of propylene oxide. Thus, when potassium hydroxide is used to catalyze propylene oxide polymerizations, the product contains allyl alcohol-initiated, monofunctional impurities. As the molecular weight of the product polyether increases, the isomerization reaction becomes more prevalent. Consequently, poly(propylene oxide) products having equivalent weights of about 800 or more prepared using KOH as the catalyst tend to have very significant quantities of the monofunctional impurities. This tends to reduce the average functionality and broaden the molecular weight distribution of the product.
More recently, the so-called double metal cyanide (DMC) catalysts have been used commercially as polymerization catalysts for alkylene oxides. Because some of these catalysts do not significantly promote the isomerization of propylene oxide, polyethers having low unsaturation values and higher molecular weights can be prepared, compared to potassium hydroxide-catalyzed polymerizations.
These DMC catalysts are described, for example, in U.S. Pat. Nos. 3,278,457, 3,278,458, 3,278,459, 3,404,109, 3,427,256, 3,427,334, 3,427,335 and 5,470,813, among many others. The composition of these catalysts can vary widely, but can generally be represented by the formula
Mb[M1(CN)r(X)t]cxc2x7zLxc2x7aH2Oxc2x7nMxAy
wherein M is a metal ion that forms an insoluble precipitate with the metal cyanide grouping Mxe2x80x2(CN)r(X)tand which have at least one water soluble salt;
M1 is a transition metal ion;
X represents a group other than cyanide that coordinates with the M1ion;
L represents an organic complexing agent;
A represents an anion that forms a water-soluble salt with M ion;
b and c are numbers that reflect an electrostatically neutral complex;
r is from 4 to 6; t is from 0 to 2; and
z, n and a are positive numbers (which may be fractions) indicating the relative quantities of the complexing agent, water molecules and MxAy, respectively.
However, experience has shown that most of the possible combinations of M, M1, X, L, r and t do not provide a catalyst having sufficient activity to be of commercial interest. Most combinations show virtually no activity at all. In addition, not all of those possible combinations of M, M1, X, L, r and t provide very low unsaturation poly(propylene oxide) polymers. Recently, developmental and commercial efforts have focussed almost exclusively on zinc hexacyanocobaltate, together with a specific complexing agent, t-butanol.
Zinc hexacyanocobaltate (together with the proper complexing agent and an amount of a poly(propylene oxide)) has the advantages of being active and of not significantly catalyzing the propylene oxide isomerization reaction. Because of the activity of this catalyst, it can be used in such small amounts that it is less expensive to replace it than to recover it from the product polyether. As a result, finishing operations such as are commonly used to remove alkali metal hydroxides from polyethers can be avoided, thereby reducing the overall production cost. However, as the catalyst is often left in the product, it must be replaced. Thus, it would be desirable to reduce the cost of the catalyst as much as possible consistent with obtaining efficient polymerizations and desirable products.
As described in U.S. Pat. No. 5,470,813, one disadvantage of DMC catalysts is that they tend to require an induction period of close to an hour to many hours in some cases before becoming active. Little polymerization occurs during this induction period. For some operations, it would be desirable to reduce this induction period.
It would be desirable, therefore, to provide an active catalyst for polymerizing alkylene oxides, which is less expensive to prepare than zinc hexacyanocobaltate complexes. It would be even more desirable to provide a catalyst that exhibits a short induction period before rapidly polymerizing alkylene oxides.
In one aspect, this invention is a metal hexacyanocobaltate nitroferricyanide catalyst complexed with one or more organic complexing agents, wherein the metal is any that forms a water-insoluble precipitate with hexacyanocobaltate and nitroferricyanide groups, and the molar ratio of hexacyanocobaltate to nitroferricyanide groups is from about 20:80 to about 98:2.
In another aspect, this invention is a method of making an active polymerization catalyst, comprising (a) forming a first solution of water soluble hexacyanocobaltate and nitroferricyanide compounds, said hexacyano-cobaltate and nitroferricyanide compounds being present in proportions such that said solution contains a molar ratio of hexacyanocobaltate to nitroferricyanide ions of about 20:80 to about 98:2,
(b) mixing said first solution with a second solution of a water soluble salt of a metal that forms a water-insoluble precipitate with hexacyanocobaltate and nitroferricyanide groups, in an amount sufficient to provide a stoichiometric excess of the metal salt relative to the combined amounts of hexacyanocobaltate and nitroferricyanide ions contained in said first solution, so as to precipitate a metal hexacyanocobaltate nitroferricyanide, and
(c) either simultaneously or after step (b), contacting said metal hexacyanocobaltate nitroferricyanide with an organic complexing agent.
In a third aspect, this invention is an improvement in a process for polymerizing an epoxide compound in the presence of a catalyst, in which the catalyst is a metal hexacyanocobaltate nitroferricyanide complexed with one or more organic complexing agents, said metal being any that forms a water-insoluble precipitate with hexacyanocobaltate and nitroferricyanide groups.
It has been found that the metal hexacyanocobaltate nitroferricyanide complex of the invention has excellent activity as an epoxide polymerization catalyst, and in particular exhibits unusually low induction periods. Because some of the expensive hexacyanocobaltate starting materials of conventional DMC catalysts are replaced with nitroferricyanide compounds, the overall cost of the catalyst is reduced significantly.
In addition, it has been found that poly(propylene oxide) polymers produced using the catalysts of this invention having extremely low unsaturation content, even when complexing agents such as glyme are used. Using materials such as glyme instead of t-butanol as a complexing agent for the catalyst provides significant advantages in preparing and handling the catalyst complex. t-Butanol tends to cause gelling and to make it difficult to isolate double metal cyanide catalysts. Thus, this invention provides a method by which low unsaturation polyethers can be prepared with a less expensive, more easily handled catalyst.
The catalyst of this invention is a metal hexacyanocobaltate nitroferricyanide that is complexed with an organic complexing agent. As used herein, hexacyanocobaltate refers to a group having the structure [Co(CN)6]3xe2x88x92, where the cobalt is in the plus-three oxidation state, Co+3. Nitroferricyanide refers to the group [Fe(CN)5NO]2xe2x88x92, where the iron is in the plus-three oxidation state, Fe+3. Nitroferricyanide ion is sometimes referred to as a nitroprussidate ion.
The hexacyanocobaltate and nitroferricyanide groups are present in a molar ratio of from about 10:90, preferably from about 20:80, more preferably from about 30:70, to about 98:2, preferably to about 95:5, more preferably to about 70:30.
The metal (designated M below) is one which forms an insoluble precipitate with hexacyanocobaltate and nitroferricyanide ions, and which has at least one water-soluble salt. M is preferably a metal ion selected from the group consisting Zn+2, Fe+2, Co+2, Ni+2, Mo+4, Mo+6, Al+3, V+4, V+5, Sr+2, W+4, W+6, Mn+2, Sn+2, Sn+4, Pb+2, Cu+2, La+3 and Cr+3. M is more preferably Zn+2, Fe+2, Co+2, Ni+2, La+3 and Cr+2. Zn+2 is especially preferred.
Preferably, a stoichiometric excess of the metal is present, relative to the combined number of equivalents of hexacyanocobaltate and nitroferricyanide groups. This excess metal is believed to exist in the form MxAy, where A is an anion that forms a water-soluble salt with M, and x and y are numbers that reflect an electrostatically neutral salt.
The catalyst is complexed with an organic complexing agent. A great number of complexing agents are potentially useful, although catalyst activity may vary according to the selection of a particular complexing agent. Examples of such complexing agents include alcohols, aldehydes, ketones, ethers, amides, nitriles, sulfides, and the like.
Suitable alcohols include monoalcohols and polyalcohols. Suitable monoalcohols include methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol, octanol, octadecanol, 3-butyn-1-ol, 3-butene-1-ol, propargyl alcohol, 2-methyl-2-propanol, 2-methyl-3-butyn-2-ol, 2-methyl-3-butene-2-ol, 3-butyn-1-ol, 3-butene-1-ol, 1-t-butoxy-2-propanol and the like. Suitable monoalcohols also include halogenated alcohols such as 2-chloroethanol, 2-bromoethanol, 2-chloro-1-propanol, 3-chloro-1-propanol, 3-bromo-1-propanol, 1,3 -dichloro-2-propanol, 1-chloro-2-methyl-2-propanol as well as nitroalcohols, ketoalcohols, ester-alcohols, cyanoalcohols, and other inertly substituted alcohols.
Suitable polyalcohols include ethylene glycol, propylene glycol, glycerine, 1,1,1-trimethylol propane, 1,1,1-trimethylol ethane, 1,2,3-trihydroxybutane, pentaerythritol, xylitol, arabitol, mannitol, 2,5-dimethyl-3-hexyn-2,5-diol, 2,4,7,9-tetramethyl-5-decyne-4,7-diol, sucrose, sorbitol, alkyl glucosides such as methyl glucoside and ethyl glucoside, and the like. Low molecular weight polyether polyols, particular those having an equivalent weight of about 350 or less, more preferably about 125-250, are also useful complexing agents.
Suitable aldehydes include formaldehyde, acetaldehyde, butyraldehyde, valeric aldehyde, glyoxal, benzaldehyde, toluic aldehyde and the like. Suitable ketones include acetone, methyl ethyl ketone, 3-pentanone, 2-hexanone and the like.
Suitable ethers include cyclic ethers such as dioxane, trioxymethylene and paraformaldehyde as well as acyclic ethers such as diethyl ether, 1-ethoxy pentane, bis(betachloro ethyl) ether, methyl propyl ether, diethoxy methane, dialkyl ethers of alkylene or polyalkylene glycols (such as ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and octaethylene glycol dimethyl ether), and the like.
Amides such as formamide, acetamide, propionamide, butyramide and valeramide are useful complexing agents. Esters such as amyl formate, ethyl formate, hexyl formate, propyl formate, ethyl acetate, methyl acetate, triethylene glycol diacetate and the like can be used as well. Suitable nitriles include acetonitrile, proprionitrile and the like. Suitable sulfides include dimethyl sulfide, diethyl sulfide, dibutyl sulfide, diamyl sulfide and the like.
Compounds having an Sxe2x95x90O group, such as dimethyl sulfoxide and sulfolane, are also useful complexing agents.
Preferred complexing agents are 1-t-butoxy-2-propanol, polyether polyols having an equivalent weight of about 75-350, DMSO, sulfolane and dialkyl ethers of alkylene and polyalkylene glycols. Especially preferred complexing agents are 1-t-butoxy-2-propanol, polyether polyols having an equivalent weight of 125-250, DMSO and a dimethyl ether of mono-, di- or triethylene glycol. 1-t-Butoxy-2-propanol, DMSO and glyme (1,2-dimethoxy ethane) are especially preferred.
In addition, the catalyst complex contains a quantity of water that is bound into the crystalline lattice of the complex.
In addition to the water and organic complexing agent, it has been found that catalyst preparation is sometimes easier if the catalyst is treated with a polyether polyol of a molecular weight of about 300-4000.
The catalyst complex is conveniently made by dissolving water-soluble hexacyanocobaltate and nitroferricyanide compounds in water and combining the resulting solution(s) with an aqueous solution of a water-soluble metal salt. The hexacyanocobaltate and nitroferricyanide solutions can be made individually, or as a single solution. If made separately, the hexacyanocobaltate and nitroferricyanide solutions are either blended before being added to the solution of the metal salt, or else are added simultaneously to the solution of the metal salt. Upon mixing the solutions, a metal [hexacyanocobaltate nitroferricyanide] complex precipitates. The precipitate is treated with an organic complexing agent of the type described above, and optionally is treated with a polyether polyol as well.
Any water-soluble hexacyanocobaltate and nitroferricyanide compounds be used. Preferably, the compounds are those represented by the structures B3[Co(CN)6] and B2[Fe(CN)5NO], where B is hydrogen or an alkali metal. For the hexacyanocobaltate compounds, B is preferably sodium or potassium. For nitroferricyanide compounds, B is preferably hydrogen, sodium or potassium and most preferably hydrogen. Compounds in which B is hydrogen are conveniently formed by passing an aqueous solution of the corresponding alkali metal salt through a cation-exchange resin that is in the hydrogen form.
The metal salt is any water-soluble compound that will react with the hexacyanocobaltate and nitroferricyanide compounds to form a metal [hexacyanocobaltate nitroferricyanide] precipitate. The metal salt suitably is one represented by the structure MxAy, where M, A, x and y are as defined before. Suitable anions A include halides such as chloride and bromide, nitrate, sulfate, carbonate, cyanide, oxalate, thiocyanate, isocyanate, isothiocyanate, and C1-4 carboxylate. Chloride ion is especially preferred.
Especially suitable metal salts include zinc halides, zinc hydroxide, zinc sulfate, zinc carbonate, zinc cyanide, zinc oxalate, zinc thiocyanate, zinc isocyanate, zinc C1-4 carboxylates, and zinc nitrate. Zinc chloride, zinc acetate and zinc nitrate are most preferred.
The catalyst complex is precipitated by mixing the solution of the metal salt with the solution(s) of the hexacyanocobaltate and nitroferricyanide compounds. The temperature of mixing is not critical provided that the starting materials remain in solution until the nixing is performed. Temperatures of about 10 to about the boiling temperature of the solution, particularly about 15-35xc2x0 C., are most suitable. The mixing can be done with rapid agitation. Intimate mixing techniques as are described in U.S. Pat. No. 5,470,813 can be used, but are not necessary.
In this initial mixing, at least a stoichiometric amount of the metal salt is used. In one variation, a stoichiometric excess of metal salt is used in the initial mixing. In another variation described more below, only a stoichiometric amount of metal salt is used in the initial mixing, with additional metal salt being added in a later step. By xe2x80x9cstoichiometric amountxe2x80x9d, it is meant an amount sufficient to form an electrostatically neutral precipitate with the hexacyanocobaltate and nitroferricyanide ions, with substantially no excess of the metal salt being used.
The complexing agent, either neat or as an aqueous solution, typically is added before significant precipitation of the metal [hexacyanocobaltate, nitroferricyanide] occurs, generally by adding the complexing agent immediately after mixing the solutions of metal salt and hexacyanocobaltate and nitroferricyanide. If desired, the complexing agent can be mixed into the metal salt solution or the hexacyanocobaltate and nitroferricyanide solution, or both. After adding this initial amount of complexing agent, the mixture is generally stirred for several minutes to allow the desired catalyst complex to form and precipitate.
The resulting precipitated catalyst complex is then recovered by a suitable technique such as filtration. Preferably, the catalyst complex is subjected to one or more subsequent washings with water, complexing agent, polyether polyol (when used) or some combination thereof. This is conveniently done by re-slurrying the catalyst in the liquid with agitation for several minutes and filtering. Washing is preferably continued at least until essentially all unwanted ions, particularly alkali metal and halide ions, are removed from the complex.
When a polyether polyol is used in the catalyst complex, it can be added with the initial amount of complexing agent, or in one or more subsequent washings of the complex.
The final catalyst complex is conveniently dried, preferably under vacuum and moderately elevated temperatures (such as from about 50-60xc2x0 C.) to remove excess water and volatile organics. Drying is preferably done until the catalyst complex reaches a constant weight.
In an alternative technique for forming the catalyst complex, an aqueous solution containing only a stoichiometric amount of metal salt in relation to the combined amount of hexacyanocobaltate and nitroferricyanide compounds is used in the initial mixing and precipitation step. After the initial precipitation is complete, the precipitate is washed with water to remove unwanted ions. The precipitate is then combined with a small amount of a solution containing water, additional metal salt, and complexing agent. The metal salt used may the same as that used in forming the precipitate, or may be a salt of a different metal. The amount of this added solution is preferably that amount which is absorbed by the precipitate. A typical amount of solution to be used is from about 0.5, preferably to about 0.8, more preferably from about 1, to about 2, preferably to about 1.5, milliliters of solution per gram of isolated precipitate. The amount of metal salt added with this solution is advantageously about 9, preferably from about 11 to about 30, preferably to about 25, parts by weight per 100 parts by weight of the precipitate. The complexing agent is advantageously present in a weight ratio of about 90:10 to about 10:90, preferably about 70:30 to about 30:70, with the water. If desired, a polyether polyol can be included in the solution. The resulting catalyst complex can be dried and used without further treatment, or may be subjected to additional washings with water as before, although it is preferred not to perform additional washings with complexing agent or polyether polyol.
In addition, the catalyst may be formed directly as a dispersion in an initiator compound, as described in copending application Ser. No. 09/574,847, filed May 19, 2000, entitled Method for Preparing Metal Cyanide Catalyst/Polyol Initiator Slurries.
Furthermore, it is also possible to form a filterable and/or supported catalyst by precipitating the catalyst in the presence of a polycarboxyl or polycarboxylate compound, as described in copending application Ser. No. 09/574,848, filed May 19, 2000, entitled Method for Preparing Metal Cyanide Catalysts using Polycarboxylic Acids, or in the presence of a silane-functional complexing agent, as described in the copending application Ser. No. 09/574,842, filed May 19, 2000, entitled Method for Preparing Metal Cyanide Catalysts using Silane-functional Ligands.
The catalyst complex of the invention is used to polymerize alkylene oxides to make polyethers. In general, the process includes mixing a catalytically effective amount of the catalyst with an alkylene oxide under polymerization conditions, and allowing the polymerization to proceed until the supply of alkylene oxide is essentially exhausted. The concentration of the catalyst is selected to polymerize the alkylene oxide at a desired rate or within a desired period of time. Generally, a suitable amount of catalyst is from about 5 to about 10,000 parts by weight metal cyanide catalyst per million parts combined weight of alkylene oxide, and initiator and comonomers, if present. More preferred catalyst levels are from about 10, especially from about 25, to about 500, more preferably about 100 ppm, most preferably about 50 ppm, on the same basis.
For making high molecular weight monofunctional polyethers, it is not necessary to include an initiator compound. However, to control molecular weight, impart a desired functionality (number of hydroxyl groups/molecule) or a desired functional group, an initiator compound is preferably mixed with the catalyst complex at the beginning of the reaction. Suitable initiator compounds include monoalcohols such methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol, octanol, octadecanol, 3-butyn-1-ol, 3-butene-1-ol, propargyl alcohol, 2-methyl-2-propanol, 2-methyl-3-butyn-2-ol, 2-methyl-3-butene-2-ol, 3-butyn-1-ol, 3-butene-1-ol and the like. Suitable monoalcohol initiator compounds include halogenated alcohols such as 2-chloroethanol, 2-bromoethanol, 2-chloro-1-propanol, 3-chloro-1-propanol, 3-bromo-1-propanol, 1,3-dichloro-2-propanol, 1-chloro-2-methyl-2-propanol and 1-t-butoxy-2-propanol as well as nitroalcohols, keto-alcohols, ester-alcohols, cyanoalcohols, and other inertly substituted alcohols. Suitable polyalcohol initiators include ethylene glycol, propylene glycol, glycerine, 1,1,1-trimethylol propane, 1,1,1-trimethylol ethane, 1,2,3-trihydroxybutane, pentaerythritol, xylitol, arabitol, mannitol, 2,5-dimethyl-3-hexyn-2,5-diol, 2,4,7,9-tetramethyl-5-decyne-4,7-diol, sucrose, sorbitol, alkyl glucosides such as methyl glucoside and ethyl glucoside and the like. However, the catalyst tends to perform better when the initiator is a polyether polyol, particularly one having an equivalent weight of about 350 or less, more preferably about 125-250.
Among the alkylene oxides that can be polymerized with the catalyst complex of the invention are ethylene oxide, propylene oxide, 1,2-butylene oxide, styrene oxide, and mixtures thereof. Various alkylene oxides can be polymerized sequentially to make block copolymers. More preferably, the alkylene oxide is propylene oxide or a mixture of propylene oxide and ethylene oxide and/or butylene oxide. Especially preferred are propylene oxide alone or a mixture of at least 50 weight % propylene oxide and up to about 50 weight % ethylene oxide.
In addition, monomers that will copolymerize with the alkylene oxide in the presence of the catalyst complex can be used to prepare modified polyether polyols. Such comonomers include oxetanes as described in U.S. Pat. Nos. 3,278,457 and 3,404,109 and anhydrides as described in U.S. Pat. Nos. 5,145,883 and 3,538,043, which yield polyethers and polyester or polyetherester polyols, respectively. Hydroxyalkanoates such as lactic acid, 3-hydroxybutyrate, 3-hydroxyvalerate (and their dimers), lactones and carbon dioxide are examples of other suitable monomers that can be polymerized with the catalyst of the invention.
The polymerization reaction typically proceeds well at temperatures from about 25 to about 150xc2x0 C., preferably from about 80-130xc2x0 C. A convenient polymerization technique involves mixing the catalyst complex and initiator, and pressuring the reactor with the alkylene oxide. Polymerization proceeds after a short induction period, as indicated by a loss of pressure in the reactor. Induction periods of from less to one minute to about 20 minutes are commonly seen, and induction periods are often less than 15 minutes. Once the polymerization has begun, additional alkylene oxide is conveniently fed to the reactor on demand, until enough alkylene oxide has been added to produce a polymer of the desired equivalent weight.
Another convenient polymerization technique is a continuous method. In such continuous processes, an activated initiator/catalyst mixture is continuously fed into a continuous reactor such as a continuously stirred tank reactor (CSTR) or a tubular reactor. A feed of alkylene oxide is introduced into the reactor and the product continuously removed.
The catalyst of this invention is especially useful in making propylene oxide homopolymers and random copolymers of propylene oxide and up to about 15 weight percent ethylene oxide (based on all monomers). The polymers of particular interest have a hydroxyl equivalent weight of from about 800, preferably from about 1000, to about 5000, preferably about 4000, more preferably to about 2500, and unsaturation of no more than 0.02 meq/g, preferably no more than about 0.01 meq/g.
The product polymer may have various uses, depending on its molecular weight, equivalent weight, functionality and the presence of any functional groups. Polyether polyols so made are useful as raw materials for making polyurethanes. Polyethers can also be used as surfactants, hydraulic fluids, as raw materials for making surfactants and as starting materials for making aminated polyethers, among other uses.
The following examples are provided to illustrate the invention, but are not intended to limit its scope. All parts and percentages are by weight unless otherwise indicated.